1. Field of the Invention
The present invention relates to a thermal head control circuit and a thermal head control method for controlling a thermal head for developing color on thermosensitive recording paper, and more particularly to a thermal head control circuit and a thermal head control method permitting multicolor printing.
2. Description of the Prior Art
FIG. 1 illustrates the configuration of a conventional thermal head control circuit. In the figure, a thermal head has a plurality each of heating resistor elements 600a to 600c and transistors 700a to 700c. A print control section 100 receives print information from a main control section, such as the control unit of a printer (not shown), supplies a head control section 200 with print data, indicating whether or not each dot on one line should be or need not be printed, as well as a latch signal S2. The head control section 200, in accordance with the print data, turns on or off the transistors 700a to 700c respectively connected to the heating resistor elements 600a to 600c constituting individual dots of the thermal head.
Next will be described the operation of this prior art thermal head control circuit.
The print control section 100, having completed outputting of print data indicating individual dots on one line to the head control section 200, supplies the latch signal S2 to the head control section 200. The head control section 200 has a shift register for shifting, for instance, print data supplied from the print control section 100, latches the print data in accordance with the latch signal S2, and outputs them in parallel. This causes the transistor, out of the transistors 700a to 700c, corresponding to each dot supposed to be printed out of the print data, to be turned on. A current flows to a heating resistor element through the transistor turned on, and the heating resistor element emits heat. Accordingly, each dot to be printed on the thermosensitive recording paper is printed.
The following description refers to a case in which such a thermal head control circuit is applied to a small size thermal printer and dichroic printing is accomplished. Dichroic printing is usually accomplished by varying color development according to a difference in calorific value (energy) given to thermosensitive recording paper.
The calorific value (energy) is proportional to the duration of the electrification of the heating resistor elements. Therefore, when second color development is to be accomplished in dichroic printing, a greater calorific value needs to be applied than in first color development, and the transistors 700a to 700c need to be kept electrified for a longer duration. In this case, where high speed printing is to be done, the duration of electrification may prove too long to be accommodated in one line step of recording paper feeding. This problem can be solved, as described in the U.S. Pat. No. 4,983,054, by reducing the paper feeding speed for the second color development.
Incidentally, not only in dichroic printing, color development can be controlled by regulating the duration of electrification in usual adjustment of density as well.
The conventional thermal head control circuit, being composed as described above, involves the problem that the printing speed is reduced especially when printing in multiple colors. Moreover, when multicolor printing has to be done on a dot-by-dot basis within one line, a plurality of different durations of electrification should be handled, resulting in complexity of processing by the head control section.
An object of the present invention is to provide a thermal head control circuit and a thermal head control method permitting, where multicolor printing is to be performed, printing at constant speed all the time without suffering a drop in printing speed.
According to the invention, there is provided a thermal head control circuit for developing at least two colors on a thermosensitive paper by providing heat on the thermosensitive paper on a dot-by-dot basis. The thermal head control circuit is provided with a thermal head having a plurality of heating resistor elements, for providing heat on the thermosensitive paper on the dot-by-dot basis; a print data circuit for generating print data indicating whether or not each of the individual heating resistor element in the thermal head is to be electrified irrespective of the color to be developed; a color development data circuit for generating color development data indicating which color should be developed on the dot-by-dot basis; an electrifying circuit for electrifying each of the individual heating resistor element according to the print data; and a current switching circuit for switching a magnitude of current flowing to the electrifying circuit according to the color development data to develop the colors on the thermosensitive paper.
The current switching circuit has the current limiting element for limiting the current flowing through the respective heating resistor elements in the thermal head, and a control circuit responsive to the color development data for controlling whether or not each current limiting element should be electrically connected to the corresponding heating resistor element.
Also the current switching circuit may has the current limiting elements for limiting the currents flowing through the respective heating resistor elements in the thermal head, and a control circuit responsive to the color development data for controlling the switching of the current limiting elements to be electrically connected to the corresponding heating resistor element.
A thermal head control method according to the invention, for controlling a thermal head to print on thermosensitive recording paper permitting dichroic printing, is a method to prepare color development pattern data indicating color development for each of dots constituting one line, set a state in which a limiting resistor is connected to a heating resistor element corresponding to data indicating color development in a lower temperature range in a color development data pattern, and set a state in which a limiting resistor is not connected to a heating resistor element corresponding to data indicating color development in a higher temperature range in the color development data pattern.
As described above, according to the present invention, since the thermal head control circuit and the thermal head control method are so configured as to set a state in which a limiting resistor is connected to a heating resistor element corresponding to data indicating color development in a lower temperature range in a color development data pattern, and set a state in which a limiting resistor is not connected to a heating resistor element corresponding to data indicating color development in a higher temperature range in the color development data pattern, the calorific values of the heat resistor elements can be regulated even if the duration of electrification is kept constant, multicolor printing can be accomplished at constant speed all the time.